1. Field of the Invention
The present invention relates to an optical semiconductor device having an optical waveguide which decreases a waveguide loss at a bent portion thereof.
2. Description of the Related Art
There has been a method for forming an optical waveguide immediately under the surface of a silicon substrate by a SIMOX (Separation by Implanted Oxygen) method. This method is characterized in that a portion to be used as an optical waveguide can be formed inside a single crystalline silicon material while the surface thereof is allowed to remain in a flat state (for example, see Prakash Koonath, Koichiro Kisima, Tejaswi Indukuri, and Bahram Jalali “Sculpting of three-dimensional nano-optical structures in silicon” Applied Physics Letters Vol. 83, No. 24, pp. 4904 to 4911, Dec. 15, 2003). Hence, this method has particularly drawn attention since an optical integrated circuit and an electrical integrated circuit can be integrated in a three-dimensional manner in one silicon substrate.
As one structural example of an optical waveguide formed by an SIMOX method, as shown in FIG. 12, the structure is formed in which a silicon layer 113 is formed on a silicon substrate 111 with a first silicon oxide layer 112 interposed therebetween, and a second silicon oxide layer 114 is formed in this silicon layer 113. This silicon layer 113 between the first silicon oxide layer 112 and the second silicon oxide layer 114 is a first silicon layer 115 forming an optical waveguide, and a part of the first silicon layer 115 having a large thickness, that is, a part of the first silicon layer 115 at which a part of the first silicon oxide layer 112 is formed to protrude to the silicon substrate 111 side, is to be used as an optical waveguide 121. In this case, the second silicon oxide layer 114 is formed to have two flat surfaces. In addition, a part of the silicon layer 113 on the second silicon oxide layer 114 is a second silicon layer 116. In order to form the structure described above, after a mask (not shown), which has an opening in a region in which the above optical waveguide 121 is to be formed, is formed on the silicon substrate 111, for example, oxygen is ion-implanted to form the first silicon oxide layer 112, and after the above mask is removed, for example, oxygen is further ion-implanted, thereby forming the second silicon oxide layer 114. Accordingly, the optical waveguide 121 is formed from the first silicon layer 115 so as to have a ridge shape.
In addition, as shown in FIG. 13, the silicon layer 113 is formed on the silicon substrate 111 with the first silicon oxide layer 112 interposed therebetween. In this silicon layer 113, the second silicon oxide layer 114 is formed, and a part of the silicon layer 113 (first silicon layer 115) provided between the first silicon oxide layer 112 and the second silicon oxide layer 114, which is a region to be formed into the optical waveguide 121, is formed to have a thickness larger than that of the other region. In addition, a part of the silicon layer 113 on the second silicon oxide layer 114 is the second silicon layer 116. Accordingly, since the first silicon layer 115 and the second silicon layer 116 are formed from the same layer, after a mask (not shown) is formed on this silicon layer 113 in a region in which the above optical waveguide is to be formed, for example, when oxygen is ion-implanted, the second silicon layer 116 is formed, and the optical waveguide 121 is formed from the first silicon layer 115 to have a ridge shape. That is, when oxygen is ion-implanted in the silicon layer 113 of an SOI (silicon on insulator) substrate, the optical waveguide 121 having the above structure can be formed.
In recent years, proposals have been made to form a MOS device on the surface of a silicon substrate having an optical waveguide therein which is formed by an SIMOX method or the like (for example, see Tejaswi Indukuri, Prakash Koonath, and Bahram Jalali “Three-dimensional Integration of metal-oxide-semiconductor transistor with subterranean photonics in silicon” Applied Physics Letters Vol. 88, 121108-1-3, 2006). As the optical waveguide formed inside a silicon substrate by this SIMOX method, a ridge-type optical waveguide has been reported. However, since a ridge-type optical waveguide generally has not strong light confinement, when it is compared with a strip-type optical waveguide, an optical waveguide loss is disadvantageously increased when the optical waveguide is bent.